As disclosed in U.S. Pat. Nos. 4,117,057 (Yajima et al.) and 4,560,526 (Okumura et al.), it is known that ceramics can be prepared from mixtures of SiC powder and a preceramic binder and that the strengths of the ceramics decrease with decreasing density. An unfortunate result of this strength/density relationship is that ceramics having typical densities (i.e., about 2.0-2.5 g/cc) have generally not been obtainable with sufficiently high bending strengths (i.e., at least about 20, preferably at least about 28 kg/mm.sup.2) to make them commercially attractive.
A Shin Nisso Kako Co., Ltd. product specification sheet, "SiC Products by PPMC Process," indicates that ceramics having a density of 2.3 g/cc and bending strengths of 20-30 kg/mm.sup.2 can be obtained from a mixture of SiC powder, polysilastyrene, and an organic lubricant. However, independent measurements of disc specimens provided by Shin Nisso show the bending strengths of these ceramics to be only about 18-21 kg/mm.sup.2 and the specific strengths, i.e., the bending strength/density ratios, to be only about 9.6-11.1 (kg/mm.sup.2)/(g/cc). This represents an improvement over the strengths of previously known low density SiC ceramics but not as much of an improvement as might be desired.
U.S. Pat. Nos. 4,482,669 (Seyferth et al.-I), 4,645,807 (Seyferth et al.-II), 4,650,837 (Seyferth et al.-III), and 4,659,850 (Arai et al.) disclose the utility of polysilazanes as preceramic materials, and Seyferth et al.-I teach that their polysilazanes are especially useful as binders for ceramic powders such as SiC. SiC ceramics prepared from these SiC powder/polysilazane binder compositions are composed predominantly of SiC, e.g., about 80-90% SiC, with the balance being mostly silicon nitride. Seyferth et al.-I do not disclose the typical densities and bending strengths of ceramics made from their preceramic compositions. However, Wiseman, "The Development and Application of Polysilazane Precursors to Ceramics," a Massachusetts Institute of Technology thesis, 1984, shows that these densities were generally about 2.0-2.4 g/cc and the bending strengths were poor. Wiseman shows a recognition, though, of its being desirable to minimize alkali metal contamination and to use preceramic polymers having a sufficiently high molecular weight, or mixtures (such as 80/20 mixtures) of such polymers with lower molecular weight polymers, to maximize strength.
Copending application Ser. No. 120,102 (Semen et al.), filed Nov. 13, 1987, teaches that high strength/low density SiC ceramics can be prepared from the polysilazanes of Seyferth et al.-I when mixtures of the polysilazanes with SiC powders have particle sizes not larger than about 105 micrometers, i.e., when the SiC/polysilazane particles are small enough to pass through a 105-micrometer sieve. However, even though the as-pyrolyzed strengths of the ceramics obtained from these preceramic compositions are good, it would be desirable to improve them, as well as to improve the strengths of ceramics derived from preceramic compositions having larger particle sizes.